The communication protocol based on the current V.34 Recommendation and T.30 Recommendation ANNEX-Fuses a full-duplex modem up to the control channel and a half-duplex modem for the primary channel which transmits image information. In this way, it switches over between these modems. FIG. 1 shows the communication protocol.
In FIG. 1, phase 1 is a section of exchange between a CM signal (calling menu signal) and JM signal (common menu signal). This section selects a modulation mode available to the calling and answer modems. Phase 2 is a section called "line probing." L1 and L2 each includes simultaneous transmissions of 21 single-frequencies from 150 Hz to 3750 Hz, and are used to probe the amplitude characteristic of the line viewed from the answer modem. INFO is a communication capacity information signal and A, A, B, and B indicate the ACK (Acknowledge) signals that receive INFO.
Phase 3 is a primary channel preparation stage which transmits V.34 image signal data which will be described later and corresponds to the period of transmission of a long training signal (long synchronization signal). The frequency band (or symbol rate) is determined based on the probing result of the line amplitude characteristic of the L1 and L2 signals in phase 2.
The control channel includes signals represented by C and D. Part C is the section in which parameters for the modem itself are mainly exchanged, determining the signal rate of the image signal data transmitted based on a modulation system called "primary channel" which comes later. Part D is a section to exchange control information as the facsimile terminal and is controlled according to commands such as DIS (Digital Identification Signal) and DCS (Digital Command Signal) described in the T.30 Recommendation.
A signal called "MPh (Modem Parameter Exchange)" of part C contains a bit (bit 50 in MPh) which determines whether or not to accept asymmetric transmission rates of 1200 bps and 2400 bps as the transmission rate of part B in the same control channel and is further provided with another bit (bit 27 in MPh) which requests the answer modem to transmit part B at either 1200 bps or 2400 bps.
At present, the T.30 ANNEX-F does not accept asymmetric transmission rates in FAX transmissions by setting MPh bit 50 to "0". When a symmetric rate transmission is selected, if the requested transmission rate differs between the calling and answer modems, transmission of part D is designed to be carried out according to the lower rate side. In a current FAX which incorporates a V.34 modem, the transmission rate of part D of this control channel is preset to either 1200 bps or 2400 bps.
The primary channel includes facsimile image information data which is transmitted by a modulation system called "V.34 primary channel" and a short training signal (short synchronization signal) which precedes the facsimile image information data. The data signal rate of this image information part is determined by the transmission/reception of the aforementioned MPh signal of part C of the control channel.
FIG. 2 is a flowchart that shows a switchover from the end of the control channel program to the primary channel on the answer modem. FIG. 2 shows the operation of the control channel in FIG. 1 when the answer modem transmits a CFR signal (CONFIRMATION TO RECEIVE signal: based on T.30 Recommendation), then a Flag pattern (a series of "7E"s in hexadecimals: based on T.30 Recommendation) and waits for all "1" signals of at least 40 bits from the calling modem.
Since each symbol of the control channel of the V.34 Recommendation consists of 2 bits or 4 bits, the answer modem normally judges data for every short bits such as 2 bits, 4 bits or 8 bits. Therefore, it judges whether the data is all "1" or not for every 8 short bits (S1). Furthermore, it judges whether all "1" receive signals consist of at least 40 bits or not according to the ANNEX-F of the T.30 Recommendation (S2). When it is confirmed that the receive signal contains a series of "1"s for at least 40 bits, the Flag signal that was being transmitted is cut and transmitted with a turn-off sequence of the V.34 Recommendation (all "1" signal in a short period) added and then signal transmission is stopped (S3).
In response to this, the calling modem waits until the carrier from the answer modem turns off, and stops transmission of all "1" signals, adds a turn-off sequence, then turns off signal transmission. The answer modem monitors the carrier from the calling modem turn off (S4) and if it confirms that this carrier turns off, then closes the reception of the control channel and switches to the reception of the primary channel (S5).
However, since the switching method above switches the reception mode after detecting that the carrier from the calling modem turns off, it has the problem of easily missing the switching timing. In addition, this method of switching to the receive mode after detecting that the carrier turns off, no matter how secure it may appear, actually has a disadvantage of the modem having difficulty in catching the signal-off section if line noise of -43 dBm or greater exists on the line. Without detecting this signal-off section, the switching of the primary channel is impossible, making it impossible to receive image information data. The T.4 Recommendation stipulates that a facsimile apparatus should operate normally when the receive signal level ranges 0 dBm to -43 dBm and the transmit output should be made adjustable from -15 dBm to 0 dEm.
One of the methods to switch from the control channel to the primary channel is a method in which the answer modem continues reception and demodulation irrespective of whether the carrier from the calling modem turns on/off, detects the timing at which all "1" receive data is changed to data other than all "1" data and then switches to the reception of the primary channel. However, if a considerable amount of delay is introduced into the line, this system has the problem of provoking switching timing errors as shown below in the case of communications with overseas for instance, resulting in a failure in the reception of the primary channel.
FIG. 3 is a timing chart that shows the terminating timing of the control channel assuming that a delay of 50 ms has occurred. If there is no line delay, the answer modem turns off its carrier when it confirms 40 bits of all "1" signals from the calling modem, and thus the calling modem also detects the carrier-off and stops transmission of all "1". The answer modem detects the stoppage of transmission and switches to the primary channel. This switching is performed with the timing of E.
In the case of a delay of 50 ms, if the answer modem confirms 40 bits of all "1" signals and turns off the carrier, the calling modem detects it 50 ms after point E and turns off the transmission of all "1". It is at point F, 50 ms later, that this transmission-off is detected on the answer modem. Therefore, the answer modem switches to the primary channel at this point F.
As shown above, if there is a large amount of delay in the circuit and great attenuation in both directions on the line, for example, if there is a great level difference between the transmission level of -15 dBm and incoming level of -43 dBm, the answer modem is likely to erroneously switch from the control channel to the primary channel with the timing of E.
FIG. 4 shows a transmission wraparound at the answer modem and the receive signal from the calling modem. The signal having a width of "a" indicates an all "1" transmit signal from the calling modem and shows considerable attenuation due to circuit attenuation. The signal having a width of "b" indicates a wraparound signal of the Flag signal on the answer modem and is a large signal because it is not attenuated. The ratio b/a is as great as 20. If there is no circuit attenuation, "a" approximates to "b". For example, if the transmit signal from the answer modem becomes a near-end echo of the exchange and returns to the answer modem without attenuation, the amplitude of the wraparound signal from the answer modem may grow approximately 20 times in size compared to the amplitude of the incoming signal from the calling modem at the input of the A/D converter.
With the timing of G in FIG. 4, the carrier is turned off after a 40-bit signal is received, and therefore the transmission wraparound from the answer modem is cut off. This reduces the amplitude considerably from "b" to "a", making it easier to produce bit errors. That is, waiting for the all "1" signal to terminate while simply checking the all "1" signal from a point just before G in FIG. 4 is likely to switch to the reception of the primary channel with the timing of G erroneously. Switching with this timing will receive the control channel signal (all "1" signal) in receive mode of the primary channel. This timing error will result in a failure in the reception of the primary channel.